In a relay apparatus on a network, concentration of traffic or the like causes congestion. Especially, in wireless networks, congestion is likely to occur because of increased real-time streaming services and a narrower band of transmission channel in comparison with wired networks. The representative systems of wireless networks include the wireless LAN (IEEE 802.11) and cellular (3rd Generation Partnership Project (3GPP)).
FIG. 10 is a view illustrating an example structure of a wireless LAN system. As illustrated in FIG. 10, the wireless LAN includes a media server 101, a wireless network gateway 102, an access point (AP) 103, and a wireless terminal 104.
The media server 101 stores video data for providing a real-time streaming service for the wireless terminal 104, for example. The data stored in the media server 101 is delivered through the wireless network gateway 102 and the AP 103, to the wireless terminal 104.
FIG. 11 is a view illustrating an example structure of a cellular network. As illustrated in FIG. 11, the cellular network includes a media server 111, a gateway 112, the Internet 113, a serving/gateway general packet radio service support node (xGSN) 114, a radio network controller (RNC) 115, a base station 116, and a wireless terminal 117.
The media server 111 stores video data for providing a real-time streaming service for the wireless terminal 117, for example. The data stored in the media server 111 is delivered through the gateway 112, the Internet 113, the xGSN 114, the RNC 115, and the base station 116, to the wireless terminal 117.
In multimedia services, or especially in video services, distribution on the Internet or the like places a large load on the network, and a massive amount of video data puts great pressure on the storage. To avoid these problems, the video data is compressed by a compression coding method, as typified by MPEG1 or MPEG2, for example. Coding methods for 3G cellular phones include H.263 and H.264. H.264 was a newly proposed coding method allowing the compression rate to be changed in a wide band from the Quad Common Intermediate Format (QCIF) level to the high definition (HD) level and was integrated with the Advanced Video Coding (AVC), which is part 10 of MPEG-4, to provide H.264/AVC.
If the media server delivers streaming data to a wireless terminal, a relay apparatus such as a wireless network gateway relays data from the media server and sends the data to the wireless terminal. If the wireless network gateway resends data in a wireless section because of degraded wireless quality, the data received from the media server waits to be transmitted, causing a data jam.
If a plurality of wireless terminals request communication with a wireless network in the same period, congestion occurs, limiting wireless resources (such as time) that can be used by the terminals and causing a data jam in the wireless network gateway. If different channels are used in the 3GPP, for example, the relay apparatus is not affected by the simultaneous access of the wireless terminals. In High Speed Downlink Packet Access (HSDPA), since a channel is shared, a data jam will occur. In the wireless LAN, because of the shared channel and half-duplex communication, data transmission opposite in direction to streaming data transmission will also be affected.
FIG. 12 is a view illustrating a data jam. Data 121 in FIG. 12 is data a relay apparatus such as a wireless network gateway, for example, receives from a media server. Data 122 indicates a data transmission state in a wireless section. Data 123 indicates data received by wireless terminals.
The relay apparatus receives data sent from the media server to the wireless terminals #0 to #2, as indicated by the data 121. Because the wireless section is a shared channel, transmission data contention occurs. Therefore, the output of the relay apparatus will cause a data jam.
FIG. 13 is a view illustrating restrictions on video data reproduction. Data 131 in FIG. 13 indicates data sent by a sending station, which is a media server. Data 132 indicates data sent from a relay apparatus to a wireless terminal, by relaying the data 131 of the sending station. The data 132 sent from the relay apparatus to the wireless terminal has been delayed by congestion, in comparison with the data 131 sent by the sending station. Shaded parts of the data 131 to 133 indicate reference pictures, and non-shaded parts indicate non-reference pictures.
The data 133 reaches the wireless terminal at the timing when the data can be reproduced appropriately. The wireless terminal can reproduce the data 131 sent by the sending station appropriately in real time if it can receive the data 131 of the sending station within the range of a permissible delay amount indicated by an arrow A101 in FIG. 13.
The data 132 relayed and sent by the relay apparatus is sent with a delay exceeding the permissible delay amount, as shown in FIG. 13. Therefore, the data 132 received by the wireless apparatus is late for the reproduction time, by the period of time indicated by an arrow A102 in FIG. 13.
A transmission apparatus and a transmission program that control media data transmission of one party without affecting delay changing in media data transmission of the other party have already been provided (refer to Japanese Laid-open Patent Publication No. 2006-140984, for example).